New research shows that people who are at risk for rheumatoid arthritis are much more likely to develop the disease if they smoke cigarettes.

In a new study, Swedish scientists have analyzed the interaction between the major genetic risk factor for rheumatoid arthritis—a gene mutation called the shared epitope—and the environmental risk factor smoking.

Among those with the gene mutation, smokers were more likely to develop rheumatoid arthritis than nonsmokers. The take home point is that smoking and the gene mutation are a dangerous combination. Smokers with the gene mutation were five to fifteen times more likely to have rheumatoid arthritis than people at no risk for disease.

“The most important thing is that we can see this very strong gene/environment interaction,” says Lars Alfredsson of the Karolinska Institute in Stockholm. He co-led the study with his colleague Lars Klareskog, also at Karolinska.

The study was large and included 858 patients with rheumatoid arthritis and over 1,000 healthy people. It took advantage of a nationwide program in Sweden that began in 1996 with the goal of diagnosing and treating people with arthritis early-on.

The findings, Alfredsson says, should give people, especially those with family members with rheumatoid arthritis, one more reason not to smoke.